Because that is all this is. A chair. The thing itself has no power, only that which they give to it, and for Obi-Wan is means nothing at all. For Rey it could be the sort of thing that might cause her to stumble, but only that.

Because it's merely a chair.

"Come," he agrees. "Let's get dry and if you want, we can talk about it more. Or we can decide how to get rid of it."

He thinks a lightsaber or two should be able to take it apart.